1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device. More particularly, the invention relates to imaging lenses which are built in image pickup devices mounted in highly functional products such as smart TVs and 4K TVs, information terminals such as game consoles and PCs, and mobile terminals such as smart phones, mobile phones and PDAs (Personal Digital Assistants).
2. Description of the Related Art
In recent years, highly functional products, such as a smart TV as a TV with a personal computer function and a 4K TV as a TV with four times higher resolution than a full high-definition TV, have been attracting attention. In smart TVs, the tendency toward products which are not only highly functional but also multifunctional is growing, so the smart TV market is expected to expand in the future. Some smart TVs provide a function to take video and still images through a built-in image pickup device and transmit the images through a communication network. This function can be used in various application fields: for example, a video phone and a high-precision people meter based on face recognition technology, and other various products such as a security product and a pet monitoring product which have a moving object detection function. Also, due to its high resolution, a 4K TV can reproduce an image which is so realistic as if the object were there. With the spread of smart TVs or similar products, these functions are expected to be more popular than before. On the other hand, digital photo frames with a camera function have been recently introduced into the market. Thus, the market related to cameras is expected to expand.
In communications over a video phone, for example, in a TV conference in which several people participate, the facial expression of the speaker and the surrounding scene are important information. In addition, when face recognition technology is used to recognize the faces of human beings or animals, image recognition should be highly accurate. The imaging lens used in such a high resolution product is required to have a compact lens system which provides high resolution, a wide field of view and high brightness.
However, in the conventional techniques, it is difficult to meet this demand satisfactorily. For example, the image pickup device used in a highly functional product such as a smart TV is assumed to adopt a relatively large image sensor suitable for high resolution images. If a conventional imaging lens is used in a large image sensor, since its optical system should be large, the following problem arises that various aberrations become more serious and it is very difficult to achieve the same level of high optical performance as in a small image sensor. In addition, when the lens is designed to provide a wide field of view, correction of aberrations may be very difficult, particularly in the peripheral area, regardless of image sensor size and it may be impossible to deliver satisfactory optical performance.
As an imaging lens built in an apparatus with an image pickup device, the imaging lens described in Patent Document 1 (JP-A-2010-262270) or the imaging lens described in Patent Document 2 (JP-A-2012-155223) is known.
Patent Document 1 discloses an imaging lens which includes, in order from an object side, a first lens with positive refractive power having a convex shape on the object-side surface near an optical axis, a second lens with negative refractive power, a third lens with positive refractive power having a concave shape on an image-side surface near the optical axis, a fourth lens with positive refractive power having a convex shape on the image-side surface near the optical axis, and a fifth lens with negative refractive power near the optical axis. The imaging lens described in Patent Document 1 includes five constituent lenses, each of which is optimized to deliver high performance.
Patent Document 2 discloses an imaging lens which includes, in order from an object side, a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, a fourth lens group with negative refractive power, a fifth lens group with positive refractive power, and a sixth lens group with negative refractive power. In the imaging lens described in Patent Document 2, the lens configuration of the optical system is concentric with an aperture stop so as to suppress astigmatism and coma aberrations and provide a wider field of view.
The imaging lens described in Patent Document 1 has a lens system which provides high brightness with an F-value of 2.0 and a relatively wide field of view with a half field of view of about 38 degrees. However, it cannot meet the recent demand for a wider field of view. Also, for use in a large image sensor, various aberrations must be further suppressed. However, if an imaging lens uses five constituent lenses, its ability to correct aberrations is limited and it is difficult to apply the imaging lens to a higher resolution product as mentioned above.
The imaging lens described in Patent Document 2 provides relatively high brightness with an F-value of about 2.3 and can correct aberrations properly. However, its half field of view is about 33 degrees, which means that it cannot meet the demand for a wide field of view satisfactorily. Also, if the lens configuration described in Patent Document 2 is employed to provide a wide field of view, correction of aberrations will be difficult, particularly in the peripheral area and high optical performance cannot be delivered.
As mentioned above, in the conventional arts, it is difficult to provide a sufficiently wide field of view while ensuring compactness, and meet the demand for high resolution. Also, for use in a large image sensor, it is difficult to deliver the same level of high optical performance as in a conventional small image sensor.